This invention relates to high speed modems.
In a co-pending application entitled "A High Speed Modem Synchronized To A Remote Codec" Ser. No. 07/963539, filed Oct. 20, 1992 and assigned to the assignee hereof, an invention was described for employing modems that are synchronized to the A/D and D/A converter clocks of the digital network over which signals are communicated. Specifically, a modem was described which is synchronized both in time and in quantization levels to the A/D converters in the network and which also takes into account the compressions introduced in the telephone network.
The synchronizing of such modems to the telephone network permits one to greatly reduce the noise that is introduced into the signals and also allows user modems to operate at the network's clock ram. In the T1 carrier system, for example, a 64 kb/s rate can theoretically be achieved.
Current local exchange carrier (LEC) switches need to provide "plain old telephone service" (POTS) to customers, which is analog service, and also need to provide digital signals to the inter-exchange carders, such as AT&T. In order to satisfy customer needs and inter-exchange carder requirements, LEC switches include a hybrid that divides the POTS two-wire bidirectional signal into a four-wire pair of unidirectional signals. In addition, an A/D converter and a D/A converter is included on the network side (i.e., the four-wire side) of the hybrid.
One characteristic of the improved modem disclosed in the above-referenced patent application is that the signal generated by the modem takes into account the .mu.-law compression and expansion that exists in the telephone network. What that means is that the step sizes between adjacent digital signals vary exponentially.
This large variation in step sizes (e.g., by a factor of 128) presents a problem when the hybrids that are included in the LEC switches are imperfect. Imperfect hybrids allow a "leak-through" from the received signal to the transmitted signal and that, of course, is tantamount to noise.
In some equipment, the prevention of leak-through is no better than 6 db (power). What that means is that a digital signal of level 126, for example, will create an analog signal of level 3,888 units (via the .mu.-law expansion) and that analog signal level, arriving at one analog port of the hybrid, will induce a 1,944 unit analog signal in the other analog port of the hybrid. The induced analog signal adds to the true analog signal of the port, and the challenge is to separate the induced signal from the true signal. Clearly, a modem that suffers a leak-through of this magnitude will have great difficulty in dealing correctly with many of the digital levels of the corrupted signal (e.g., a digital signal of level 100 results in an analog signal of 1,300 units compared to the undesired 1,944 unit leak-through, and all other digital signals lower than level 100 result in lower analog signals).
It is recognized that the telephone network contains two hybrids in an established interconnection: a near-end hybrid between a first modem (the near-end modem) and the network, and a far-end hybrid between the network and a second modem (the far-end modem). Each modem must deal, therefore, with a near-end echo (from the modem closest to it) and a far-end echo. With respect to the near-end echo, the interference suffered by the first modem in its reception of signals is due to its own transmitted signal leaking through the near-end hybrid into the received signal. Since this path is completely analog, the echo cancelling circuitry within the modem can effectively account for and discount the leak-through. Hence, the leak-through from the near-end hybrid does not present a problem. The leak-through from the far-end hybrid (the hybrid in the LEG switch connected to the far-end modem), on the other hand, does present a problem because of the quantization and the .mu.-law compression expansion that is encountered by the signal in its path. The path includes the near-end modem, the near-end hybrid, the telephone network, leakage at the far-end hybrid, the network again and the near-end hybrid again. Because of the non-linearities in this path, the echo cancelling circuitry in the near-end modem cannot effectively cancel the transmitted signal echo.
For instance, when a digital word corresponding to the number 126 needs to be transmitted by the first, near end, modem, the analog signal that is sent by the modem is 3,888 units (.mu.-law expansion). With only 6 db attenuation in the leak-through path, this signal creates a far-end echo of analog magnitude 1,944 units. If at the time corresponding to this echo the remote modem is attempting to send digital words corresponding to the number 2, which maps to an analog level 2 units, the composite signal of analog level 1,946 units is quantized to digital level 110 and sent to the near-end modem. At the near-end LEG switch, this digital level is converted to analog level 1,944, resulting in a failure on the part of the modem to detect the digital level 2 signal that was sent by the remote modem.